Photoelectron--photoion coincidence spectroscopy of gas-phase clusters

A photoelectron--photoion coincidence technique for obtaining the photoelectron spectrum of a single component of a gas-phase mixture has been developed. It utilizes a newly designed instrument which measures the ion mass in coincidence with the photoelectron kinetic energy. Initial experiments were carried out on Xe2 and Xe3 produced in mixture of clusters (plus monomer) in a free-jet supersonic expansion. These measurements determined the photoelectron appearance potential (i.e., the lowest binding energy for which photoelectrons are detected) to be 11.30(5) eV for Xe3. It was also found that fragmentation of cluster ions strongly affects the coincidence spectra. This was investigated by varying the stagnation pressure, and thus the beam composition, in order to assess fragmentation contributions to coincidence spectra of the cluster under study. One case studied in detail indicated the energy levels of Xe3 near the ionization potential of 11.6 eV, corresponding to 0.7 eV of internal energy in Xe+3, fragmented to form Xe+2 and Xe